95% of all vendors at "gun shows" which sell guns are FFL (federal firearms licensee, legal gun dealers). Any gun sold/transfer from an FFL to a regular person (non-FFL) must have a background check and 4473 form completed. The vast majority of people attending gun shows and buying guns thus are subject to background checks.
Now there are the simple collectors of guns which show up and sell from their collection on occasion, or the guys walking around with a gun strapped to their back and a "for sale" sign in the barrel which don't need a background check, but they have become few and far between. Websites like guntrader have become like a craigslist for people who want to sell their guns face to face.
Every time I've bought a gun Face to Face I've done a bill of sale for the gun, and showed my CWP or drivers license. (it's not legal to sell hand-guns to out of state residents with out shipping it to a FFL in their state)
This "gun show loophole" is in reality a hollow argument. If a rational person actually researches the laws, you find out how hard it is to buy a gun with out a background check.